Center fed antenna



CENTER FED ANTENNA A Filed May 24,- 1945 IN V EN TOR. LEON H/MME LPatented Mar. 1, 1949 UNITED STATES PATENT OFFICE CENTER FED ANTENNALeon Himmel, Bronx, N. Y., assignor to Federal Telephone and RadioCorporation, New York, N. Y., a corporation of Delaware Application May24, 1945, Serial No. 595,514

(Cl. Z50-33) Claims.

This invention relates to antennas and more particularly to verticalantenna arrangements.

Heretofore center fed vertical dipole antennas were designed withradiator lengths which were based on the one-half wavelength as astandard unit. In certain cases like the present instance, structurallimitations made it inopportune to employ antennas of such length andthe height of the vertical antenna and of the radiators had to belimited so that it became necessary to eX- plore the possibility ofemploying an antenna based on the quarter Wavelength. It becameimmediately apparent that quarter wavelength vertical center fedantennas, especially ones which were to be used over a wide frequencyband, presented a number of diiculties and particularly if a balanceddouble conductor transmission line were utilized. In feeding a verticaldipole antenna through its center an unbalance ordinarily would becreated in the transmission line. In order to avoid this unbalance it isnecessary to introduce a shield for the transmission line which may alsobe utilized as a supporting pipe structure for the radiators. In order,however, to prevent a coupling of the radiator current with thesupporting pipe at the end away from the midpoint of the antenna, whichif left open would present a direct short-circuit to such currents, adirect conductive contact must be established between the shield and theradiator adjacent the feed, or center, portion of the antenna. Thus, alarge impedance is established at the other end between the radiatorsand the shield. Ordinarily, in the case of a single frequency and unlessa Very precise balance of the radiators is required, only an outershield will be required in the upper antenna portion. Since, however, insuch a case one of the two lines is connected to the shield, anunbalance would be created which would prevent the occurrence of currentin the lower radiator due to the unbalance of the relative impedancevalues between the line conductors and the shield. It is thereforenecessary to isolate the line from its shield by introducing anadditional shield which has a length of a quarter wave at the midpointfrequency in a wide band antenna. A large eiective impedance is therebyestablished between the line and the new shield and a zero impedance ata point which is a quarter wavelength from the point of maximumimpedance when looking from the direction of the antenna midpoint orfeed point. To establish a balance in the radiator structure,particularly when handling a wide band of frequencies, an image of 2 theshield structure may be provided in the upper portion of the antennastructure. The length of the actual radiators for proper response over adesired range of frequencies may vbe made adjustable.

It is accordingly an object of the present invention to provide avertical center fed dipole antenna which is operable over a Wide band offrequencies and presents a symmetrical radiation characteristic, thelength of the dipole being based on a quarter wave standard length andwhich is fed from a balanced dual transmission line.

In accordance with the present invention, I provide two verticallyarranged tubular radiators which are fed by a balanced dual transmissionline passing through the lower of the two radiators and which isarranged to feed the radiators at their mid or junction point. Disposedwithin the two radiators are two concentrically arranged shields theouter of which acts as a supporting structure for the antenna. The

outer shield is conductively connected to the' respective upper andlower radiators adjacent the mid or junction points thereof while theinterna-1 or second shield is itself conductively connected to the outeror iirst shield at a point which is a quarter or if desired an oddmultiple of a quarter wavelength at midfrequeney away from the nearestconductive contact between the outer shield and the radiators. Thisdistance is a quarter wavelength from the midpoint for a quarter waveantenna and for an antenna longer than a quarter wavelength for each ofits two radiators, it is an odd multiple of a quarter Wavelength fromthe open end between the outer and inner shields.

These and other features and objects of my invention will become moreapparent upon consideration of the following detailed descriptionA to beread in connection with the accompanying drawings in which:

Fig. 1 represents in diagrammatic form of an embodiment of the inventionfor an antenna having individual radiators substantially one quarterwavelength long at the midpoint frequency; and

Fig. 2 is a diagrammatic representation of an embodiment of an antennain accordance with my invention the individual radiators of which may belonger than a quarter wavelength.

Referring to Fig. 1 the antenna structure is composed of radiators l and2 which arestructurally supported by tubular shields 3 and 4 the formermember 3 being shown supported on a' porting member 3. The two radiatorsand shield.,

portions are disposed along the same aXis and held out of conductivecontact at tl'ieirradiacent vr` ends by means of an insulating member 9,disposed at the midpoint thereoi.V The twoconductors of the transmissionline may be made topass through the said insulator 9 to make Contactwith conductive portions Hl and I I which provide a connection betweenthe respective radiators and their corresponding shields and which maybe in the form of conductive and supporting discs., adjacent the twosides of the insulator 9. The transmission line conductors are furthersupplied with an additional or internal tubular shield I2 which isdisposed within the outer shield .ii-Ll and. preierably extending in onepiece from the lower portion of the outer shield 3 to the upper portionof the shield rl. Openings may be provided inthe inner shield I2 atI3'and M to permit the conductorsA 6 and. 1 to pass therethrough towardthe junction points at Hl.A and l l. The inner shield I2 is alsoprovided with a short-circuiting member i therefrom to the outer shield3 and a similar member Ir for` the. upper shield It each lata pointwhich is located overalllengths ofthe individual radiatorsmay beadjusted to a total length of. a quarter wave.- length of the maximumwavelength encountered inthe frequency band to be covered. In. order tovp rovide structural. strength, both. the inner and outer radiatorportions have been furnished with insulating members. disposed betweenthe respective radiator portions andthe outer shields or supportingtubular members- 3 and as at 2l, 22, for the upper radiator andat 23;and 24 for the lower radiator. Similarly, members may be` provided las asupport of the inner shield at 25a However, the member 25 may be omittedif desiredand theinner shield terminatedl at the con-` ductivemember I5.The upper shield. orsupport` thasE been provided with a closure orcapmember 26.

The antenna structure of Fig. 2 is similar inall respects to that shown.in F'ig. 1 except that thev length of theupper and lower` radiatingportions is a-multiple ofa quarter wavelength. where limitation inphysical length is not critical. The overall length of each radiatormaybev expressed by theV general term:

where N maybe any suitableinteger fromzero up. 'Ifheessential diierenceover `the antenna of Fig. 1 lies in thezprovision of` a, shert-circuitlrespectivelybetween radiators I and 2 and'thecorrespond..

ing outer shield or supporting tube portions 3 and i by means ofshort-circuiting portions 21 and 28 Iat a point which is a quarter or anodd multiple of a quarter wavelength away from the open outer end of theradiators and by locating the short-circuits l5-I 6 between the innershield l2 and the outer shield 3 4A at points which are away from theinner and open end of the inner shield I2.

It thus becomes apparent that in operation the antenna as describedhereinabove provides an outer quarter wave trap. and an inner quarterwave trap which are effective in preventing the appearance of radiofrequency currents on the supporting tube and on the inner shield.l Theor if desired Aeiiect is such that the radiators are tuned toapproximately a quarter wavelength with respect to the outer shieldwhereby a high impedance is seen at a point looking in at the open endof the radiators and similarly, a conductive contact between the innershield and the outer shield is provided at a point substantially aquarter wavelength from the open end of the two shields which is at theantenna midpoint. A high impedance is thus seen between said inner andouter shield.

when looking in at the open end -or feed point. Over a given range oifrequencies the impedance between inner and outer shields varies frominductive to capacitive. However, both input lines have this impedanceinshunt with theantenna to ground and therefore remain balanced. Bygivingv the coaxial section -a high surge impedance by means of highratios of diameters the antenna may be made to cover a broad band. Whilethe quarter wavelength radiators are the preferred form, it will be seenfromA the above that the principle outlined hereinabove has applicationin connection with radiators having a length which isa multiple ofquarter wavelength.

While l.' have described above the principles of my invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as la limitationon the scope of my invention as deiined inthe objects and theaccompanying claims.

I claim: l. A vertical dipole antenna comprising a pair of tubularradiators having outer ends and innerY substantially adjacent ends; abalanccddouble transmission line comprising two` conductors extending inpart thru one of said radiators, said conductors being in conductiveContact one. with each of said radiators at said inner ends; an outershield disposed coaxially within said radiators having a conductiveconnection therewith adjacent said inner ends, and an inner shieldcoaXially disposed within said outer shield having a conductiveconnection there-with a given.

distance from said inner ends.

2. An antenna according to claim l, wherein said connection comprises astructural portion -for each radiator and said outer shield constitutesa supportingstructure for said portions and said radiators.

3. An antenna according to claim 1, further including insulating` meansdisposed intermediate said inner ends.

4. An antenna according to claim 1, wherein said pair of radiatorscomprises a pair of tubular members having each an axial lengthsubstantially equal to one quarter wavelength at a representativemidfrequency.

5. An antenna according to claim 1, wherein said pair of radiatorscomprises a pair of tubular members having an axial length substantiallyequal to one quarter wavelength at the highestl frequency in a givenoperation band further including a tubular sleeve slidably secured incoaxial relation to each of said radiators, whereby the effective lengththereof may be adjusted in accordance with the operative wavelength.

6. A vertical dipole antenna comprising a pair of tubular radiatorshaving outer ends and inner substantially adjacent ends, a balanceddouble transmission line comprising two conductors extending in partthru one of said radiators, said conductors being connected one to eachof said radiators at said inner ends; outer shield means disposedcoaxially within said radiators, inner shield means coaxially disposedwithin said outer shield, rst means forming a trap for a wave of a givenlength between said radiators and said outer shield, and second meansforming a trap for a wave of a given length between said outer shieldand said inner shield.

7. An antenna according to claim 6, wherein said rst and second trapforming means form traps which are eiective for a one quarter wavelengthat a representative midfrequency.

8. An antenna according to claim 6, wherein said rst and second trapforming means form traps which are effective, for odd multiples ofquarter wavelengths at a representative midfrequency.

9. An antenna according to claim 6, wherein said rst and second meansforming traps comprise conductive connections between said radiators andsaid outer shield and between said outer and said inner shield, and higheffective impedances therebetween at the ends thereof which are notconnected.

10, A vertical dipole antenna comprising a pair of tubular radiatorshaving outer ends and inner substantially adjacent ends; a balanceddouble transmission line comprising two conductors extending in partthru one of said radiators, said conductors being connected one to eachof said radiators at said inner ends; outer shield means disposedcoaxially within said radiators conductively connected theretorespectively adjacent said inner ends, and inner shield means coaxiallydisposed within said outer shield means conductively connected thereto adistance from said inner ends which is equal to an odd multiple of aquarter wavelength at a representative midfrequency.

LEO-N HIMMEL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,111,636 Lindenblad Mar. 22,1938 2,113,136 Hansell et al. Apr. 5, 1938 2,184,729 Bailey Dec. 26,1939 2,229,733 Goldmann Jan. 28, 1941 2,267,951 Roosenstein Dec. 30,1941 2,323,641 Bailey July 6, 1943 2,385,783 Alford Oct. 2, 1945

